The present invention relates generally to thick film sensors and, more particularly, it relates to thermistors, i.e. resistive elements that are heat sensitive. Still more particularly, the invention relates to a novel thermistor structure and an infrared intrusion detector employing same.
Oxide materials having a high temperature co-efficient of resistance (TCR) are well-known. Cobaltous oxide, CoO, is one such material. It is also known that CoO in a thick film paste should be fired at a temperature not exceeding about 1350.degree. C., in order to prevent a substantial decrease in TCR.
In the field of radiation detectors generally and infrared detectors more particularly, a variety of approaches are known. Typically, an array of serially-connected thermocouples is used to form a heat-sensitive sensor. If the application of the device is to be as an intrusion detector, where it is motion of a warm body that is to be detected and a stationary body (a light bulb, radiator or the like) is not to be detected, the sensor is associated with an array of spherical mirrors and only a signal generated by successive pulses, as caused by a body moving across the field of view of the mirror and focused successively on different sensors, triggers the alarm circuitry. Ferroelectric materials such as triglycine sulfate (TGS) are also well known in heat-sensitive sensors.
An intrusion detector including a thermocouple array and mirror focusing is disclosed in U.S. Pat. Nos. 3,792,275 and 3,760,399. A photosensitive junction device is the sensor in U.S. Pat. No. 3,704,735. A silicon disc with a crossed electrode pattern is the sensor disclosed in U.S. Pat. No. 3,529,161. A thermistor made by ion implantation is disclosed in U.S. Pat. No. 3,693,011. Typical of patents using TGS ferroelectric sensors are U.S. Pat. Nos. 3,604,933 and 3,539,803. In the latter, a field effect transistor and a high-value resistor are used to lower the normally high output impedance of ferroelectric devices.
The properties and preparation of cobalt oxide for thick film sensor applications are discussed in U.S. Pat. No. 3,345,596.